This invention relates to a large size image display apparatus in which such images as television video signals or image signals are displayed on a display board including a large number of display elements.
In a conventional display apparatus of this type, a large number of incandescent lamps or a cathode ray tube is used as the display elements or element. However, such prior art display apparatus are not satisfactory in that the operating life of the lamps is relatively short, and in that the lamps consume a large amount of power and generate a large quantity of heat, thus requiring a large and expensive cooling means. Furthermore, a cathode ray tube is not only expensive, but also has difficulty producing a sufficient quantity of light.
Furthermore, an image display device utilizing discharge lamps, for example fluorescent lamps, which are a type of low pressure gas discharge lamp, has also been proposed as disclosed in Japanese Patent Publication Nos. 5894/1971 and 30797/1973. Discharge lamps have longer operating life and better light emitting efficiency than incandescent lamps and are of lower cost and can emit a larger quantity of light than can cathode ray tubes. However, in the display apparatus described in these patent publications, the discharge lamps are operated with DC voltage so that the following disadvantages arise. More particularly, the discharge lamps have a negative voltage-current characteristic so that it is necessary to use current limiting devices. When the discharge lamps are lighted with DC voltage as taught by the Patent Publications described above, it is necessary to use resistors as the current limiting devices with the result that the electric power loss and the concommitant heat generated by the resistors increase. Although the power loss and the quantity of heat generated by a resistor of a single discharge lamp are not so large, this results in a serious problem for a large size display apparatus utilizing several thousands to several tens of thousands of discharge lamps. For this reason, prior art display apparatus are not satisfactory.
When the discharge lamps are lighted with commercial AC voltage, inductors can be used as the current limiting devices whereby the problems of power loss and heat generation can be solved. This solution, however, causes the following problem. Taking a video signal of a television signal utilized in Japan as an image signal, since the video signal has a period of 1/60 second, 60 video signals are sent per second. Accordingly, in order to display such video signals, it is necessary to light each display lamp at each 1/60 second. Where the power source has a frequency of 60 Hz, it is necessary to control each discharge lamp to produce light at each cycle corresponding to the video signal. Control of the discharge lamp is necessary in order to produce a half tone color, so that where it is necessary to reproduce the video signal at a high fidelity, the number of stages of the color control must be increased. Generally, 8, 16 or 32 stages are used. Actually, the color control means is limited to phase control type. The inductor acting as the current limiting device and the load circuit of a discharge lamp can be considered as a well known L-R circuit and when AC voltage is impressed across such L-R circuit, as is well known in the art, current i expressed by the following equation flows. EQU i=Im sin (.omega.t+.theta.-.phi.)-Im sin (.theta.-.phi.) EQU .epsilon..sup.-R/.sbsp.L.multidot.t =is+it
where
.theta.: switch on phase angle PA1 .phi.: power factor angle of the circuit (tan-1.omega.L/R) PA1 is: steady current PA1 it: transient current
As can be noted from this equation, a transient oscillation occurs in the L-R circuit upon closure of a source switch so that transient current flows which attenuates with a time determined by a constant R/L. Thus, the time determined by R/L elapses before the current becomes until steady, that is steady state is reached. For this reason, when lighting discharge lamps with commercial voltage, the lamp current contains transient current at the time of applying the voltage and such transient current does not attenuate sufficiently in one cycle of the commercial source voltage. As a consequence, the lamps do not reach steady state in one cycle, thus making it difficult to provide a predetermined light control or light output. In other words, this affects the response characteristic. Where large numbers of discharge lamps and current limiting devices are used as above described, the degree of light control of respective discharge lamps would not be constant because the characteristics of the discharge lmaps and of the current limiting devices are not always the same. Such imbalance between the degrees of light control among numerous discharge lamps results in a serious problem where an image is displayed with colors. As can be noted from the equation, where the switch-ON phase angle and the circuit power factor angle coincide with each other no transient current flows. However, as a large number of stages are used for light control and an extremely large number of discharge lamps are used, it is impossible to make equal the switch-ON phase angle and the power factor angle. Thus, where many discharge lamps are operated by a commercial AC source voltage, it is impossible to obtain a desired light output corresponding to a video signal in synchronism therewith. There is another problem that the degrees of light control varies between discharge lamps.